1. Field of the Invention
This invention relates to a trifluorobenzene derivative used as a component of liquid crystal materials. More particularly it relates to a trifluorobenzene derivative suitable as a component of liquid crystal compositions useful for a passive mode and an active matrix mode, each requiring a high reliability, and a liquid crystal composition containing the derivative.
2. Description of the Related Art
Liquid crystal compositions used for liquid crystal display elements of conventional passive modes such as liquid crystal compositions used for liquid crystal display elements of 90.degree. twisted nematic mode, guest-host mode, supertwisted mode, SBE mode, etc. and further liquid crystal compositions used for liquid crystal display elements of active matrix modes such as liquid crystal compositions used for non-linear two-terminal elements of diode, etc. or three-terminal elements of thin film transistors (TFT), etc., as a switching element, are mostly liquid crystal mixtures consisting of several kinds of liquid crystal compounds and compounds similar to liquid crystals, which compounds have a CN group at the terminal group or at the side chain thereof to exhibit a positive or negative dielectric anisotropy (hereinafter abbreviated to .DELTA..epsilon.). The reason for this is that the dielectric anisotropy of the compounds, induced by the CN group, has a large positive value when the CN group is present at the terminal group, while it has a large negative value when the group is present at the side chain. For example, in the case of liquid crystal display elements of 90.degree. twist mode occupying the most part of commercially available liquid crystal display elements, the threshold voltage in the electricity-capacity characteristic is given by the following equation containing the dielectric anisotropy value .DELTA..epsilon. and elastic constants K.sub.11, K.sub.22 and K.sub.33 : ##EQU1##
Thus, it can be understood that when a compound having a large positive .DELTA..epsilon. value, i.e. a compound having a CN group at its terminal group is contained in a composition, the resulting threshold voltage Vc is reduced.
On the other hand, similarly in the other modes, the threshold voltage value depends upon the .DELTA..epsilon. value. Namely, the larger the .vertline..DELTA..epsilon..vertline., the less the Vc. As to conventional liquid crystal compositions, by adjusting the content of compounds having a CN group at the terminal end or at the side chain thereof in the above-mentioned various modes, it has been possible to drive the display element at a low voltage of several volts, which is one of the largest specific features of liquid crystal display elements.
In recent years, as the use application range of liquid crystal display elements has broadened, there has been an increasing requirement liquid crystal display elements which reduce the current consumption of liquid crystal compositions, raise the specific resistivity thereof, raise the reliability thereof such as reduction in the rate of change with lapse of time and enhancing the display contrast, in the passive mode and the active matrix mode. However, a highly polarizable group such as a CN group, in spite of its contribution to the dielectric anisotropy, has raised a problem in the aspect of the above-mentioned current consumption, specific resistivity, their rate of change with lapse of time and the resulting display contrast.
While the reason has not yet been clarified by persons skilled in the art, it may be presumed that the CN group at the terminal group or at the side chain of compounds has a certain interaction with ionic impurities present in the display element, thereby having a bad influence upon the current value, specific resistivity value and the resulting display contrast. The drawbacks of compounds having such CN group, consist in, in the case of the passive mode, causing increase in the current consumption and reduction in the specific resistivity to lower the reliability in the elements, and in the aspect of display characteristics, causing display unevenness and contrast reduction, and further, in the case of the active display elements, having a larger influence upon reduction in the reliability of display elements than that in the case of the passive display elements, coupled with the influence due to the drive current of two-terminal or three-terminal elements, thereby causing increase in the current consumption and reduction in the specific resistivity. In particular, the reduction in the specific resistivity often has a further secondary bad effect of reducing the display contrast.
At present, however, most compositions containing no CN group at the terminal group or at the side chain thereof are insufficient in the aspect of the drive voltage among the various specific features of the compositions. Thus, the low voltage drive as one of the advantages of liquid crystal display elements is often not made the best use of. Recently, in order to solve such problems, a number of compounds having one or two halogen atoms, particularly one or two fluorine atoms in the aspect of viscosity, at the terminal group or at the side chain thereof have been announced.
Examples of these compounds are as follows: ##STR2##
In these formulas, R represents a linear alkyl group.
Most of these compounds have been reported to be liquid crystal materials having a relatively good stability and a high reliability, but they have a relatively small .DELTA..epsilon. value of about +3 to +5, and liquid crystal compositions consisting only of these compounds have a small .DELTA..epsilon. value to make it impossible to reduce drive voltage.